The invention relates to a method for manufacturing a glass panel for a display tube comprising a first stage of press-forming molten glass put in a mold using a plunger and a second stage of cooling the formed glass after it has been taken out from the mold.
In the known methods a glass panel is press-formed which usually takes place at very high temperatures (1000xc2x0 C.-1100xc2x0 C.). In this manner a glass face panel can be formed. Cathode ray tubes, for example, comprise such a glass panel.
Cathode ray tubes (CRT""s) are becoming of ever more greater size, thus increasing the weight of the CRT""s. Furthermore the front surface of the glass panel is becoming ever more flatter. However, increasing the flatness of the front surface of the face panel generally increases also the weight of the glass panel because the thickness of the glass panel has to be increased to ensure safety against implosion or explosion of the CRT.
There exists a great need for increase in strength of the CRT, and in particular of the glass panel, without increasing the weight. An increase in the strength of the glass panel may improve the yield.
The present invention is aimed at providing a method which enables increasing the yield of the method and/or reducing the weight of the glass panel.
To this end the method in accordance with the invention is characterized in that during a part of the first stage after pull back of the plunger the heat radiation of the inner face portions of the central panel portion is reduced to reduce a temperature gradient to the edge portions of the panel.
The invention is based on the insight that during the cooling phase after forming in the mold inhomogeneities in the stress level in the panel can occur. In order to improve the strength of (Real Flat) display panels the cooling velocity after press forming is usually increased to a such extent that a compressive stress is produced in the surface in the order of 3 to 25 Mpa. Trying to correct for the above inhomogeneities in the stress level during a final annealing stage would easily affect the necessary compressive stress (bulk stress). This reduces the yield and can seriously affect the safety of the tubes. This is in particular important for panels with an (almost) flat inner and/or outer surface such as Real Flat panels.
Because real flat panels (for instance 51RF) have a wedge, normally the North position of the panels has a tensile stress at North position when the panel is cooled down. The intention of the inventive cooling process is to minimize temperature differences between center and edge of the front of the panel. By this the tensile stress at the North position will decrease, so cracks starting at the North position will more likely propagate to the corners instead of the center; this is better for safety. The local reduction in cooling can be advantageously realized by placing a heat reflection means opposite to the central portion of the panel which heat reflection means leaves the edge portions free. What has been explained with respect to the North position (the central area of the upper long side of the panel) also holds, mutatis mutandis, with respect to the East, South, West -and corner-positions.
Reducing the heat radiation of the thinner, central portion, of the panel has been found to be an effective means to reduce temperature gradients over the panel surface. Radiation transport is responsible for 40-70% of the heat transport. Cooling with air of other portions alone is less efficient. This effect may for instance be used in practice to manufacture panels with a lower weight, or panels with a flatter front surface, or to reduce the fall-out (=percentage of panels that does not pass safety tests) or any combination of these beneficial effects.
According to a first embodiment before removing the panel from the mold, a heat reflection means is arranged in a position facing the inner portion of the central panel portion.
According to a further embodiment a heat reflection means is used which comprises at least one plate made of a material selected from the group comprising Ni, Al, Au, or Al-oxide, or a steel plate coated with such a material.
To increase the effect of the heat reflection means an edge cooling means (fluid cooling) may be arranged adjacent at least one of the edge portions of the glass panel. (The edge portions are the areas where the viewing window and the peripheral side walls of the glass panel join.) This edge cooling assist the heat reflection means in rendering the temperature distribution over the panel face more equal.
In view of the above it is in particular an aspect of the invention to provide a display panel which has a central portion which is substantially thinner than the edge portions and which presents substantially no tensile stresses in the central area of at least one of the long and short edge portions of the panel.
A manner to realize this is to take measures that during cooling down in the forming mold all panel positions pass Tg substantially at the same moment. (Tg is the transition temperature at which the glass changes from the viscous state to the solid state.)
It is known that the contour of the inner face of panels of the type described become distorted during the cooling of the panels immediately following the forming thereof. Heretofore, a correction of such distortion was usually accomplished by directing a stream of aeriform cooling fluid to a selected area of the panels prior to removal of the panels from their forming molds, such stream of cooling fluid causing differential cooling of the panels which apparently counteracts the causes of undesirable distortion. There may be situations that the inventive heat radiation control is not compatible with the above method of correcting contours on the press.
According to a further aspect of the invention directing a stream of cooling fluid to a selected area of the panel for correcting the inner face contour is carried out after removal from the mold, during transport (e.g. on a conveyor belt) to a next processing stage (e.g. annealing in an oven).
These and other aspects of the invention will be apparent from and elucidated with reference to the embodiments described hereinafter.